The bones inside your body make a hard frame called the skeleton. Bones hold up your body and give it its shape, and a grown-up's skeleton has 206 bones. Bones also protect you. The bones of your skull protect your brain, and your ribs form a cage that protects your heart and lungs. Bones may look like dry sticks, but bones are living tissue, and a living bone has a lot going on inside it.
Bone is a framework of the protein collagen made hard by the mineral calcium phosphate. The outside of a bone is made of hard, dense bone. Bones also store calcium, a mineral, and give some back to the blood when the body needs it. Soft bone marrow inside many bones makes most of your blood cells, and red blood cells are made in the red marrow inside bones. Bone-building cells (osteoblasts) and bone-removing cells (osteoclasts) balance each other to keep bone healthy. Old bone is constantly broken down and replaced by new bone, a process called remodeling.
Long bones grow longer at growth plates near their ends, not from the middle. A child's growth plates harden into solid bone once the child is fully grown, and then the bones stop getting longer. Bones get stronger when you use them by being active. The skull grows too. A baby's skull bones are not firmly joined at birth, which leaves soft spots on the head, and as a child grows, the skull bones that cover the brain fuse together at joints called sutures.
A joint is a place where two or more bones meet. Hinge joints, like the knee and elbow, only bend and straighten, while ball-and-socket joints, like the hip and shoulder, can move forward, backward, sideways and in circles. Cartilage covers the ends of bones in a joint and helps them move without rubbing, and ligaments are tough bands that connect one bone to another. Most muscles are attached to bones, and the bones act as levers that let body parts move. Tendons connect muscles to bones.
Not everything stiff in your body is bone. The part of your ear you can see is made of cartilage, a tough bendy tissue, not bone. A shark has a skeleton made of cartilage instead of bone. A giraffe has seven bones in its neck, the same number as a person. A bird's bones are hollow and light, which helps it fly. So skeletons come in more than one kind.
How do we know about bones we cannot touch? In 1895, Wilhelm Röntgen found X-rays by accident while experimenting with vacuum tubes. His X-ray picture of his wife's hand showed the bones of her hand and her ring. Doctors use X-rays to see whether a bone is broken, and a cast or splint holds a broken bone still while it heals. Old bones can tell a story too. Lucy, a famous early human relative found in Ethiopia, has a skeleton that is about 40 percent complete. She got her nickname from a Beatles song that was playing in the team's camp. She is about 3.2 million years old.
Words to know
- osteoblasts: bone-building cells
- osteoclasts: bone-removing cells
- remodeling: old bone replaced
- sutures: skull joints
- ligaments: bone-to-bone bands
